Explosive fracturing of petroleum bearing formations

ABSTRACT

This is a method for increasing the permeability of an underground formation adjacent a well bore by explosive fracturing. In one embodiment an explosive slurry, having selected fracturing fluid characteristics, is used to hydraulically fracture the formation and such explosive slurry is subsequently detonated. In another embodiment a sorbent solvent slug is injected down the tubing preceding the injection of the explosive slurry and a second such sorbent solvent immediately follows the explosive slurry. This avoids trapping of air or other gas and therefore prevents premature ignition or detonation of the explosive fracturing fluid.

United States Patent Clarence R. Fast;

George C. Howard; Morton A. Mallinger, all of Tulsa, Okla.

[21] Appl. No. 875,843

[22] Filed Nov. 12, 1969 [45] Patented Dec. 28, 1971 [73] Assignee AmocoProduction Company Tulsa, Okla.

[72] Inventors [54] EXPLOSIVE FRACTURING OF PETROLEUM BEARING FORMATIONS8 Claims, 4 Drawing Figs.

52 us. Cl 166/290, 102/23, 166/299 [51] Int. Cl E2lb 43/26 [50] Field ofSearch 166/299,

[56] References Cited UNITED STATES PATENTS 3,561,532 2/1971 Fletcher etal 166/299 2,481,422 9/1949 Haynes et al. 166/70 X 2,708,876 5/1955Nowak 166/299 2,892,405 6/1959 Chesnut 166/299 2,892,406 6/1959 Hradelet al. 102/23 3,075,463 1/1963 Eilers et al. 166/299 3,191,678 6/1965l-linson 166/299 3,336,982 8/1967 Woodward et a1 166/299 OTHERREFERENCES Frac Shot, Richmond, Va., Reynolds Metals Co., 1961 PrimaryExaminerlan A. Calvert Attorneys-Paul F. Hawley and John D. GassettABSTRACT: This is a method for increasing the permeability of anunderground formation adjacent a well bore by explosive fracturing. Inone embodiment an explosive slurry, having selected fracturing fluidcharacteristics, is used to hydraulically fracture the formation andsuch explosive slurry is subsequently detonated. In another embodiment asorbent solvent slug is injected down the tubing preceding the injectionof the explosive slurry and a second such sorbent solvent immediatelyfollows the explosive slurry. This avoids trapping of air or other gasand therefore prevents premature ignition or detonation of the explosivefracturing fluid.

36\ NITROGEN EXPLOSIVE SORBING SOLVENT PATENTEU DEE28I97| I 3530.2 1

SHEET 1 UF 3 6\ NITROGEN EXPLOSIVE 49 54 52 I SORBfIENG SOLV NT 5 7O 6422 66 67 23 72 WATER IO s I2 3% E 1;; CLARENCE R. FAST g GEORGE c.HOWARD MORTON A. MALLINGER INVENTORS FIG. I ATTORNEY PATENTEDBEB28|97|3,830,281

SHEET 2 OF 3 EXPLOSIVE SORBING N SOLVENT 23 WATER E 29 30 A 62 mi l c gQ I 2 CLARENCE R. FAST GEORGE C. HOWARD MORTON A. MALLINGER INVENTORSATTORNEY SHEET 3 BF 3 FIG.

CLARENCE R FAST GEORGE C. HOWARD MORTON A. MALLINGER INVENTORS ATTORNEYPATENTED [H28 191:

EXPLOSIVE FRACTURING OF PETROLEUM BEARING FORMATIONS BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to a methodof increasing the permeability in a formation adjacent an oil and gaswell which penetrates therethrough. It relates especially to a system inwhich an explosive fracturing fluid is used as a hydraulic fracturingfluid and is subsequently detonated. It also relates to ways ofpreventing premature ignition of the explosive fracturing fluid and asafer way of igniting the explosive fluid.

2. Setting of the Invention Many oil and gas wells are drilled intopetroleum bearing fonnations which have low permeability. That is, theformations are of a character that is difficult for the oil to berecovered; in other words, the formation is very reluctant to give upits petroleum. When such formation rocks are encountered, it becomesnecessary to resort to one or more means to increase the permeability.One means which has been quite successful is the hydraulic fracturingtechnique. In this system a special hydraulic fracturing fluid isinjected through the well bore adjacent to the formation. Fluid iscontinued to be pumped into the well bore so as to increase the pressureto the point where the formation is fractured. When this fracturingoccurs there are usually propping agents left in the fractures so thatthe fractures will remain open, forming new paths for the oil to flowfrom the rock into the well bore. Frequently, this type stimulation doesnot result in desired production increases as it does not reach theunfractured matrix of the formation. Although hydraulic fracturing haspermitted the production of billions of barrels of oil not heretoforerecovered, there is still some room for improvement.

BRIEF DESCRIPTION OF THE INVENTION An explosive slurry having fracturingfluid characteristics is injected into the well bore at a pressure andrate sufficient to fracture the formation. This explosive fracturingfluid is detonated in situ deep within the formation rock. In additionto the fracture caused by an explosive fluid acting as a fracturingfluid, many side fractures are formed when an explosive is detonated.Special precautionary steps are also taken to avoid trapping of air orother displacement gas which sometimes causes premature ignition ordetonation of the explosive fluid. This is accomplished by injecting aslug of sorbent solvent, which is a desensitizer for the explosive used,immediately preceding the injection of an explosive slurry and a secondsuch slug immediately following the injection of the explosive slurry.

DESCRIPTION OF THE DRAWINGS Various objects and a better understandingof the invention can be had from the following description taken inconjunction with the drawings.

FIG. 1 is a vertical view, partly in section and partly schematic, of awell penetrating an underground producing formation.

FIG. 2 illustrates the lower end of a well bore which penetrates aproducing formation in which the explosive liquid has penetrated theformation.

FIG. 3 illustrates stemming in a well in which an explosive liquid hasbeen placed under pressure.

FIG. 4 is an enlarged view of one plug used in the tubing of FIGS. 1, 2and 3.

DETAILED DESCRIPTION OF THE INVENTION Attention is first directed toFIG. 1 which illustrates schematically a well equipped to carry out thisinvention. Shown thereon is a well bore which penetrates a producingformation 12. An upper part of borehole 10 is cased with a casing 14cemented in place in a conventional manner. In this particular setting,casing 14 does not extend through the producing formation l2. Suspendedwithin casing 14 is a string of tubing 16 having a sub 18 at the bottomand a removable plug insertion head 21 on top containing a conductorcable-packing gland 58. Sub 18 has plug catcher 20 on the insidethereof. As will be explained more fully hereinafter, catching means 20permits plug 22 to pass therethrough but catch plug 24. Plug 22 is shownretained in the upper part of the tubing by plug retainer pin 23 and isjust immediately ahead of the explosive fluid. Plug 24, retained by plugretainer pin 25, trails the fracturing fluid. The details of plugs 22and 24 and catching means 20 will be described in detail hereinafter.The lower end of the annulus between tubing string 16 and casing 14 issealed by packer 26 which can be a drillable production packer. Theupper or main part of tubing string 16 can be disconnected from sub 18by simply rotating and lifting. This, as will be seen, is useful forplacing cement in the well bore above packer 26.

A pipeline 29 having valve 30 therein connects the annulus 32 of thewell bore with a pit or tank not shown. This is useful in passing afluid such as cement slurry into the casing 14 above packer 26.

Attention will now be directed briefly toward that part of the equipmentlocated on the surface which is used for injecting the liquid explosiveinto the well bore. This includes a container or source 34 having anexplosive slurry or liquid therein. A nitrogen source 36 is connectedthrough line 38 having valve 40 to drive the liquid explosive fromcontainer 34. The outlet of container 34 is connected through line 42 toa wellhead manifold 49. Line 42 has a valve 48 adjacent container 34.The outlet of pump 44 is connected through line 46 to tubing string 16through manifold 49. Pump 44 is a highpressure pump such as a triplexplunger positive displacement pump capable of exerting sufficientpressure and rate to fracture formation 12. A sorbent solvent sourceindicated as container 50 is provided. This has an outlet line 52 whichconnects into line 64 leading to pump 44. Line 52 has valve 54 thereinadjacent container 50. A water source 62 connects through conduit 67 toconduit 64. Conduit 64 is connected to pump 44.

An electric-conducting line 56 extends through a packing 58 in the upperend of tubing string 16. This line 56 extends downwardly through topplug 24 and is connected to and supports an igniter or detonator 60.

Characteristics of Liquid Explosive In a preferred embodiment we proposeto use the explosive liquid itself as the fracturing fluid. SUch fluidshould have a fracturing fluid coefficient, C, of 0.01 feet per squareroot a minute, or less, (fluid coefficient, C, is a term which meansvolume lost per distance per unit of time or feet cubed/feetsquared/time in square root of minutes) and a fluid loss of less thancc. per 30 minutes (API), be essentially water immiscible and somewhatoil miscible. The water immiscibility allows the displacement of theliquid explosive down the tubing string with water. On the other hand,the oil miscibility insures desensitization of the explosive during theproduction phase. Additionally, the liquid explosive should be capableof transporting propping agents, such as sand or predominantly aluminum,into the fracture. This means that the liquid explosive should have aplastic viscosity of greater than 100 cp. This liquid explosive must beable to support detonation in the narrow confines of the fracture of aformation which may, in some cases, be less than about one thirty-secondinch. A suitable liquid explosive is one in which nitromethane is thepredominant composition and contains a thickening agent, sensitizer andother additives tailored to the bottom-hole temperature, pressure andtype formation. A suitable commercially available liquid explosive isTal-l005, which is available from Talley Industries, Inc., Mesa,Arizona. Nitromethane-based liquid explosives are especially desirableas they can be ignited and if the pressure builds up, the deflagrationwill progress to high-order detonation and unburned nitromethane willdetonate, causing the desired explosion. We show means to confine thenitromethane-based liquid explosive in the well bore. We can then use aflame igniter. The liquid explosive then can be flame squibbs and apropellant. This is much safer than a time bomb containing a highexplosive.

Operation System A sorbent solvent from container 50 is released throughline 52 with valve 54 open to line 64. This goes in through manifold 49on top of bottom plug 22. In the past, attempts to displace liquidexplosives in oil wells, resulted in the explosives being placed in thetubing string and then followed by a displacing plug. Duringdisplacement operations, air, displacing gas, or vapors from theexplosive are injected on top of the bottom plug. Under some conditions,these mixtures of air, displacement gas, and vapors when compressedunder bottom-hole temperature can cause explosives to ignite or detonateprematurely. We overcome this problem by injecting a slug of sorbentsolvent into the tubing just above bottom plug 22 and into the tubingstring just below top plug 24 after all liquid explosive has been pumpedinto the tubing string. A sorbent solvent suitable for this typeoperation is one that desensitizes the explosive and, for example, canbe dibutyl phthalate, toluene, benzene, or the like. This sorbentcombination or solvent absorbs or dissolves any gas and desensitizes anyexplosive with which it comes into contact. No compounds containing anexplosive sensitizer such as amines or acids should be used as sorbents.

Having described the suitable liquid explosives and a suitable sorbentsolvent, attention will now be directed toward their injection. Asmentioned, a slug of sorbent solvent is injected on top of plug 22. Thisvolume should be enough to occupy several feet of tubing string. At thistime valve 54 is closed and valve 48 is opened, thus connecting theliquid explosive container 34 with manifold 49. This liquid is displacedby nitrogen into the tubing string at the selected pressure and rate offlow. Continued pressuring of the liquid explosive forces the bottomplug 22 downward through the bottom end of the tubing string when itdrops down to the borehole bottom. The injection of the liquid explosivecontinues as desired. If it is desired that the liquid explosive be usedas a hydraulic fracturing fluid, then sufficient fluid is injected undersufficient pressure to cause such fracture. Quality and characteristicsof hydraulic fracturing fluid are well known. When the total amount ofliquid explosive is injected into the well, valve 48 is closed, closingoff the liquid explosive source, and valve 54 is opened and a secondslug of sorbent solvent is injected into the tubing, this time on top ofthe explosive liquid just below top plug 24. This is done by openingvalve 70 and closing valves 68 and 72. Top plug 24 is then displaceddownwardly by closing valve 54 and opening valve 66 connecting the pumpto the water source 62 and valve 68 is opened and valves 70 and 72 areclosed. after which water is pumped on top of top plug 24. When the topplug reaches locking means 20 it is locked in position which confinesthe liquid explosive in the borehole below packers 26 as shown in FIG.2.

Bottom plug 22 is a conventional cementing tubing wiper plug with a softouter elastomer body and a hard metal or plastic core. The plug isdesigned to wipe the pipe walls clean as it moves down the tubing. Thedimensions of plug 22 are such that the plug will pass through top pluglatching means 20 shown in FIG. 4.

Top plug 24, sub 18 and plug latching means 20 are shown in detail inFIG. 4. Latching means 20 is simply a reduced diameter steel ring thatis fitted in sub 18. Top plug 24 consists of elastomer wiper portions 82and 84 connected by steel center portion 86. The dimensions of centerportion 86 are such that the bottom part 88 of the center portion 86 ofplug 24 will pass through latching means 20. The top part 90 of centerportion 86 of plug 24 is preferably a metal ring which will not passthrough latching means 20. Plug 24 also contains latching ring 92 whichis a split spring steel ring that compresses on passing through latchingmeans 20 and expands after passing through so that latching ring 92bears against shoulder 94, thereby locking plug 24 in place so that itcannot move upward. Top part of the plug prevents downward movement. Topplug 24 and catcher 20 are commercially available from Baker Oil Tools,lnc., Los Angeles, California, and are identified as Model C CementingPlug and Full Flow Fill Up Collar.

As shown in FIG. 2, the liquid explosive 70 fills the borehole andextends out into fracture 72. The flame igniter or explosive detonator60 is positioned in the liquid explosive 70. It is also seen that thebottom plug 22 has dropped down to the bottom of the borehole. At thispoint, the upper part of tubing string 16 has been disconnected from sub18. At this time, cement slurry is displaced down tubing string 16 andout the lower end thereof and onto the upper side of packer 26 and upperplug 24. This placing of the cement 74 is the first step of theimportant stemming of the explosive.

Attention is next directed to FIG. 3 which illustrates the liquidexplosive extending out into the fractures filled by the liquidexplosive and in which such liquid explosive is stemmed. Show thereon isthe liquid explosive 70 with a sorbent solvent 76 just below packer 26.Above packer 26 is about feet of cement 74. It has been found that about100 feet will ordinarily be adequate. Immediately above cement 74, sand80 has been placed and extends a few hundred feet up into the casing.The remainder of the hole is filled with water to the surface. Thisstemming does two things: 1. It prevents the explosive from damaging thecasing 14 and keeps the explosive force at the formation. 2. Thisconfines the liquid explosive and in the case where the liquid explosiveis predominantly sensitized nitromethane, detonation can be accomplishedby the use of a flame igniter. Thus, when it is time to set off theexplosive composition, an electrical current is sent down wire 56 toignite the flame igniter or explosive detonator. In the case where aflame igniter is used, the buming nitromethane reverts to high-orderdetonation if the pressure generated is not relieved. As this liquid 70is confined below the stemming, the pressure is confined, thus highorder detonation occurs. There is quite an advantage to the technique ofigniting nitromethane mixtures because an igniter is much safer tohandle than a detonator, which is a high explosive initiated by adangerous blasting cap.

Tests were conducted to evaluate the use of a sorbent solvent precedingand following the injection of a flammable liquid explosive to the wellto prevent ignition. The tests were conducted in a pressure chamberfitted with an electrical heating unit and a thermocouple. A lmilliliter sample of TAL-IOOSC liquid explosive (produced by TalleyIndustries lnc., Mesa, Arizona) was placed in the chamber and gaspressure held in the chamber with the thermocouple immersed in theexplosive. Heat was applied to the chamber at a low rate and thetemperature of the explosive measured until it ignited. lgnition wasobserved by a very sharp increase in temperature. The following resultswere observed:

These tests show that at a given pressure, the ignition temperature of aliquid explosive, in this test an explosive identified as TAL-lOOSC,produced by Talley Industries, lnc., Mesa, Arizona, is appreciably lowerwith oxygen present than with nitrogen. They also show that the ignitiontemperature of the explosive decreased with increasing temperature whenoxygen was present but did not when the explosive was in a nitrogenatmosphere. These tests then suggest that a liquid sorbent should beinjected preceding and following the liquid explosion to sorb anyvolatilized explosive that is produced when the well is on vacuum duringexplosive displacement.

While the above embodiments of the invention have been described withconsiderable detail, it is to be understood that various modificationsof the device can be made without departing from the scope or spirit ofthe invention.

We claim:

1. A method of explosively fracturing a formation penetrated by a wellbore which comprises:

a. injecting a first slug of a sorbent solvent into said well bore, thesorbent solvent used in this method being capable of desensitizing saidliquid explosive which it contacts;

b. immediately following step (a) with the injection of a liquidexplosive;

c. following step (b) immediately with injection of a second slug of asorbent solvent; and

d. thereafter detonating said explosive liquid.

2. A method as defined in claim 1 including the step of stemming saidliquid explosive above said second slug of sorbent solvent.

3. A method as defined in claim 1 in which said sorbent solvent isselected from the group consisting of dibutyl phthalate, toluene andbenzene.

4. A method of using a liquid explosive to fracture an undergroundformation penetrated by a well bore in which a string of tubing issuspended which comprises:

inserting a bottom plug in said string of tubing;

placing a packer about the lower end of said string of tubing to sealthe annulus between the string of tubing and the well bore;

injecting a first slug of sorbent solvent on top of said bottom plug,the sorbent solvent used in this method being capable of desensitizingsaid liquid explosive which it contact;

injecting said liquid explosive into said tubing string on top of saidfirst slug, continuing injecting said liquid explosive until a selectedamount has been injected and driving said bottom plug through the lowerend of said tubing string; injecting a second slug of sorbent solventinto said string of tubing on top of said liquid explosive;

inserting a top plug on top of said second slug;

displacing said liquid explosive, said second slug and said top plugdown said tubing string until said top plug reaches the lower end ofsaid tubing string;

retaining said top plug in the lower end of said tubing string;

placing stemming material on top of said annulus packer and said topplug;

thereafter detonating said explosive liquid.

5. A method as defined in claim 4 wherein said step of placing stemmingmaterial comprises separating the tubing string from a lower sectionthereof and injecting cement down through said tubing string and ontothe top of said top plug and said packer; and

removing said tubing string as said cement is displaced into said well.

6. A method as defined in claim 5 in which there is about feet of cementplaced above said annulus packer and said top plug.

7. A method of explosively fracturing a formation penetrated by a wellbore which comprises:

a. injecting a liquid explosive into said well bore;

b. immediately following step (a) injecting a slug of sorbent solvent,the sorbent solvent used in this method being capable of desensitizingsaid liquid explosive which it contacts; and

c. thereafter detonating said liquid explosive.

8. A method as defined in claim 7 wherein said liquid explosive and saidsorbent solvent is directed down through a string of tubing andincluding the step of inserting a top plug in said string of tubingabove said slug of sorbent solvent.

2. A method as defined in claim 1 including the step of stemming saidliquid explosive above said second slug of sorbent solvent.
 3. A methodas defined in claim 1 in which said sorbent solvent is selected from thegroup consisting of dibutyl phthalate, toluene and benzene.
 4. A methodof using a liquid explosive to fracture an underground formationpenetrated by a well bore in which a string of tubing is suspended whichcomprises: inserting a bottom plug in said string of tubing; placing apacker about the lower end of said string of tubing to seal the annulusbetween the string of tubing and the well bore; injecting a first slugof sorbent solvent on top of said bottom plug, the sorbent solvent usedin this method being capable of desensitizing said liquid explosivewhich it contact; injecting said liquid explosive into said tubingstring on top of said first slug, continuing injecting said liquidexplosive until a selected amount has been injected and driving saidbottom plug through the lower end of said tubing string; injecting asecond slug of sorbent solvent into said string of tubing on top of saidliquid explosive; inserting a top plug on top of said second slug;displacing said liquid explosive, said seCond slug and said top plugdown said tubing string until said top plug reaches the lower end ofsaid tubing string; retaining said top plug in the lower end of saidtubing string; placing stemming material on top of said annulus packerand said top plug; thereafter detonating said explosive liquid.
 5. Amethod as defined in claim 4 wherein said step of placing stemmingmaterial comprises separating the tubing string from a lower sectionthereof and injecting cement down through said tubing string and ontothe top of said top plug and said packer; and removing said tubingstring as said cement is displaced into said well.
 6. A method asdefined in claim 5 in which there is about 100 feet of cement placedabove said annulus packer and said top plug.
 7. A method of explosivelyfracturing a formation penetrated by a well bore which comprises: a.injecting a liquid explosive into said well bore; b. immediatelyfollowing step (a) injecting a slug of sorbent solvent, the sorbentsolvent used in this method being capable of desensitizing said liquidexplosive which it contacts; and c. thereafter detonating said liquidexplosive.
 8. A method as defined in claim 7 wherein said liquidexplosive and said sorbent solvent is directed down through a string oftubing and including the step of inserting a top plug in said string oftubing above said slug of sorbent solvent.